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Modding an Ampeg SS70

Started by FleshOnGear, September 20, 2023, 01:38:56 PM

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FleshOnGear

A couple months ago I bought a solid state Ampeg SS-70. Even when it's stock the amp sounds pretty great. Channel B has a really nice clean that goes into a decent crunch at higher gain settings, and channel A does the metal thing really well. The only problems with the design are that the bass and treble controls on Ch. A are pre-distortion, Ch. A is too noisy, and the pre-distortion tone filtering takes out too much treble leaving a parked wah sounding top end (even with the treble dimed) that sounded particularly dull when rolling down the guitar's volume.

Here's the schematic:

I replaced C57 (47n) with a 4n7 cap, which opened up the top end, and replaced C20 (220n) with a 330n to extend the range of boost a bit lower. Unfortunately the extended treble caused oscillation at higher treble settings, so I wired a 10k resistor from P4's CCW tab to the wiper so that C13 drops a bit of treble at higher treble settings. Now I get oscillation only at the very highest treble and gain settings. I can probably increase C57 a little to mellow the top end a bit, but I don't run the treble very high, anyway.

I replaced IC1 and IC2 (RC4558) with very low noise LT1124 op amps. This reduced hiss significantly, even with the treble boost.

I wired a 68n cap in series with an 820 ohm resistor from the node where R19 meets R20 to ground. This drops the mids and highs relative to the bass. Before that the Ch. A tone was pretty thin, but this beefed up the overall tone a lot. I also increased C15 (560p) to 1000p, to retain a bit more presence on lower mid settings.

I'm really pleased with how this turned out. When I first played the amp I could tell it had potential, that it just needed some changes to the tone filtering before and after the distortion. I wish I could change the tone controls to be after the distortion, but it would require serious rewiring, and it's kinda cool being able to control the way the lows and highs cut through the gain. The post distortion filtering I added pretty much negates the need for post EQ, anyway. With these mods I get a distortion channel that balances well with the clean channel, and the distortion cleans up really nicely from the guitar volume.

Out of my solid state amps, this one has a unique midrange voice that I really like. My Marshall 5215 has a lot of girth to the tone, fat and corpulent; the Peavey Bandit has a smooth, creamy, but punchy midrange; this Ampeg has kind of a stiff, cutting midrange that could probably tear through any band mix. I'm a fan of all of it!

joecool85

This sounds like a great platform, but not something I'm very familiar with.  Care to share any pictures?  We'd love to see it!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Tassieviking

I could not see the schematic, so I will post the ones I have if someone is interested.
Photos would be very nice.You cannot view this attachment.You cannot view this attachment.You cannot view this attachment.   
There are no stupid questions.
There are only stupid mistakes.

FleshOnGear

#3
You cannot view this attachment.
[Edit: Ok, I figured out a way to get photos on here. I haven't had time to open up the amp again, but I'll try to get gut pics at some point.]

As far as whether or not this is a good platform, I think it's ok. The pcb has flimsy traces and it isn't silkscreened with part numbers, so you need to do detective work to find the part you intend to replace. The speaker it comes with, the Celestion G12K-85, is a bit bright in this amp. The shelved low pass I installed (the 68n in series with the 820 ohm resistor) worked well for the G12K-85, but made the amp too dark for other extension cabs. I had to change the 820 to a 1k5.

The are some big positives for this amp. The cabinet is sturdy. Even stock the clipping circuit is sufficiently tube-like in tone. And, these are relatively cheap on the used market. Just a few tweaks and I got a great, dynamic metal tone.

joecool85

Quote from: FleshOnGear on September 23, 2023, 05:47:05 PMThe are some big positives for this amp. The cabinet is sturdy. Even stock the clipping circuit is sufficiently tube-like in tone. And, these are relatively cheap on the used market. Just a few tweaks and I got a great, dynamic metal tone.

Rock and roll!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

FleshOnGear

You cannot view this attachment.
Here's a bit of the guts of the Ampeg. Please excuse the messiness of some of the work. I mounted the post distortion low pass on top of the board. All of the larger film capacitors are new; the original caps were those tiny beige tic-tac looking things.

At this point I'm not quite done. I've tried some different speakers with the amp to try and thicken the tone, with some success, but I have another speaker being shipped right now. Once I've tried that one I'm going to make a final adjustment to the shelved low pass. The goal is to get the combo sounding big enough on its own without the amp becoming boomy when plugged into my extension cabs. Right now I have an Eminence Swamp Thang in the amp, and it sounds pretty good.

I'm also considering modding the amp to include the Variable Harmonic circuitry (also known as Flexwave in Crate amps). It's just two resistors, one electrolytic cap, and one diode added to the clipping stage. I'm not sure if it will really improve the sound or feel of the amp, but it seems easy enough to try.